For an image pickup apparatus to be mounted on a cellular phone, personal computer (PC), and the like in recent years, high resolution, low cost, and small size have been strongly demanded.
A cell pitch in image pickup elements, such as CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) image sensor has been dramatically reduced, and high imaging performance with reduced optical aberration, in particular reduced axial chromatic aberration in comparison with a typical optical system has been demanded for an optical system.
At present, an optical system of a four-group/four-piece configuration has been very often adopted in high-end models for a cellular phone. Examples of such lens designs for a camera module have been disclosed in Patent Literatures 1, 2, 3, and the like.
In all of these three examples, a lens design is composed of a first lens with a positive power and a high Abbe number, a second lens with a negative power and a low Abbe number, a third lens with a positive power and a high Abbe number, and a fourth lens having a negative power on axis and a positive power off axis with a high Abbe number. Such a lens design functions to reduce an angle of a light ray incoming into an image pickup element in such a manner that the first lens to the third lens employ a triplet-type power arrangement, and the fourth lens corrects for a distortion aberration and off-axis aberration.